Understanding of several issues in methanol distillation

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　　　　　　　 Understanding of Several Issues in Methanol Distillation Xie Shusheng1, Wang Yijun2, Yan Luzhong1 (1. Lunan Fertilizer Plant, Yuanmin Coal Industry, 277527; 2. Taiyuan Coal Gasification (Group) Co., Ltd., 030024) Abstract: The principle of the three-column process for methanol distillation is briefly described, the material and heat balance relationships in the pressurized distillation column and the atmospheric distillation column are analyzed, and precautions to be taken during operation are proposed. Keywords: Methanol distillation; Pressurized column; Atmospheric column; Mass and heat balance. Article ID: 100529598 (2002) 0220044202. Chinese Library Classification Number: TQ 54. Document Code: A1. Purpose and process design of methanol distillation: The purpose of methanol distillation is to separate methanol from impurities such as water and organic substances, thereby producing high-quality purified methanol. In the selection of the methanol distillation process, the scale is often used to determine the size of the plant. Typically, methanol plants with an annual production of 40,000 tons or less (including 40,000 tons) adopt a two-tower process, while those with an annual production of over 40,000 tons use a three-tower process. The so-called two-column process refers to a setup that includes only a pre-column and a main distillation column, while the three-column process involves a pre-column, a pressurized column, and a atmospheric column. The three-column process actually divides the main distillation column in the two-column process into a pressurized column and a atmospheric pressure column. In both two-column and three-column processes, the function of the pre-column is to remove gases and low-boiling-point impurities dissolved in crude methanol, while the function of the other columns is to remove water and higher-boiling components from methanol. The three-tower process is more complex than the two-tower process, requires higher investment and presents greater operational difficulties, but its notable advantage is lower energy consumption and reduced operating costs. This is because the methanol vapor at 0.55 M Pa and 121 ℃ coming out of the pressure tower does not condense directly, as would be the case with the vapor emerging from the main distillation tower in a two-tower process; instead, it is utilized fully as a heat source for distillation in the atmospheric-pressure tower. Our plant’s annual production capacity for methanol is 100,000 tons, and a three-tower process is employed. 2 The pressurized tower and the atmospheric pressure tower represent a unified entity of materials and heat; the key aspect in designing the processes for methanol in these towers is this unity of materials and heat. In the methanol three-column distillation process, in order to make full use of energy, the methanol vapor drawn from the top of the pressurized column is used in the atmospheric pressure column. Date of receipt: 2001212219. Author’s profile: Xie Shusheng, male, born in February 1963, graduated in inorganic chemical engineering. Mainly engaged in production operation organization management and capital construction management. The heat source for the reboiler is, therefore, under certain reflux conditions, crude methanol first undergoes distillation under the control conditions of the pressure column (with a top pressure of 0.55 M Pa and a temperature of 121 °C), after which it enters the atmospheric pressure column (where the top pressure is maintained at 0.005 M Pa and the temperature is around 66 °C) to have the remaining methanol distilled. The heat source for distillation in the pressurized tower comes from the steam supplied by the bottom reboiler; the amount of pure methanol used in the reflux stream from the pressurized tower should be sufficient to provide enough heat for the reboiler in the atmospheric pressure tower. The heat source provided by the reboiler in the pressure column must not only meet the heat requirements for the distillation of crude methanol in that column, but also supply sufficient heat for the distillation process in the atmospheric pressure column. The atmospheric tower and the pressurized tower form a heat community; ensuring heat balance in the pressurized tower is the foundation for maintaining stable operation of the atmospheric tower. 3 Reasons and phenomena of system fluctuations caused by unstable heating in the pressurized tower There are many reasons that lead to operational fluctuations in the pressurized tower and the atmospheric pressure tower, as well as imbalances in materials and heat; however, changes in the heat supplied by the pressurized tower is one of the important causes. When the heat supplied by the pressure tower is insufficient, the first effect is a decrease in the pressure at the top of the tower, accompanied by a drop in temperature. This is mainly due to the heat provided by the pressure tower being insufficient to meet the needs of its own operations as well as those of the distillation in the atmospheric pressure tower. Due to insufficient heat supply within the pressurized tower, the amount of methanol vapor generated decreases; if the heat supply to the atmospheric pressure tower remains unchanged at this time, it will lead to a drop in pressure within that tower. Furthermore, due to insufficient heating in the pressure column, the crude methanol at the bottom of this column moved to the atmospheric column, increasing the load on the atmospheric column. As the load on the atmospheric tower increases, more heat is required. However, in reality, the amount of heat supplied by the pressurized tower to the atmospheric tower decreases, resulting in a further drop in the pressure at the top of the pressurized tower. Ultimately, as the heat supplied to the atmospheric tower decreases, it causes the heavier components in the tower trays to move downward, resulting in an increase in liquid level and a rise in the methanol content in the wastewater, thereby disrupting the operational equilibrium of the atmospheric tower. If adjustments are not made in such situations in a timely manner, a vicious cycle will arise. Issue 2 (Total No. 99) Coal Chemical Industry No. 2 (Total No. 99) April 2002 Coal Chemical Industry Apr. 2002 © 1994-2007 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net 4 Key points for operating the pressure column: The pressure column is the core equipment in the entire methanol distillation process; it serves as the heat source for the atmospheric pressure column. The quality of its heat balance determines the efficiency and quality of separation in distillation. In normal production, special attention must be paid to the control of the following aspects. 4.1 The pressurized tower shall ensure an adequate heat supply. The heat for the pressure tower is supplied by the reboiler at the bottom of the tank, and this heat must meet three requirements: ① It must provide the heat needed for the distillation of crude methanol entering the pressure tower, as well as for the vaporization of the reflux liquid. ②It provides the heat required for the crude methanol remaining from distillation in the atmospheric pressure tower, as well as for the gasification reflux stream. ③ While meeting the above two requirements, heating systems must have necessary adjustment mechanisms, in order to provide a means for adjusting the pressure in the pressure tower. Under ideal conditions, the heat provided by the pressurized tower not only meets the normal requirements for its own distillation process but also exactly satisfies the heat needs of the atmospheric pressure tower. But in practice, it is difficult to maintain this ideal state. Therefore, in the process design, a pressure control valve (PV 5505) for the pressurized tower and a temperature control valve (TV 5515) for the atmospheric tower were installed to ensure automatic adjustment in case of system fluctuations. Generally, the function of pressure regulation is more important; therefore, it is necessary to maintain a certain degree of opening in PV 5505. This ensures that the pressurization tower has an adequate supply of heat, and at the same time helps to reduce fluctuations in the pressure at the top of the tower during system fluctuations, thereby facilitating balance. 4.2 Control of the reflux ratio in the pressurized tower and the atmospheric pressure tower. The reflux ratio is directly related to the changes in the material concentration on each tray within the tower as well as the temperature distribution, which ultimately affects the efficiency of distillation. The selection of the reflux ratio is primarily determined by the load within the tower and the quality requirements for methanol. With other operating conditions remaining unchanged, a higher reflux ratio is beneficial for improving the purity of the methanol product, but it also requires more heat. Generally, provided that the purity of methanol is met, the reflux ratio is chosen as small as possible in order to save energy. According to the preliminary design, our plant specifies a reflux ratio of 1.5–2.0 for pressurized columns and 2.0–2.5 for atmospheric pressure columns. When the load on the tower is low, there is a surplus of tray space, allowing for a lower reflux ratio to be used. For example, when the load is less than 60%, the reflux ratios for the pressurized tower and the atmospheric tower should be around 1.5 and 2.0, respectively. When the load on the tower is high, the reflux ratio also needs to increase accordingly; for instance, when the load exceeds 90%, the reflux ratios should be around 2.0 and 2.5, respectively. The size of the reflux ratio directly determines the amount of heat supplied by the reboiler in the pressure tower. Therefore, unless the production conditions are disrupted or the product quality is unsatisfactory, under normal production conditions, once the reflux ratio is determined, it must be kept constant in order to ensure the stability of the system. 4.3 Precautions for adjusting production load and process parameters. ① In distillation operations, automatic control should be utilized as much as possible by means of existing instrumentation control methods. ②When adjusting the load of a distillation system, the magnitude of the adjustments should be as small as possible; generally, it should not exceed 10%. ③The temperature parameters for each tower are values based on the design conditions at that time; adjustments can be made according to actual circumstances, but such adjustments should be within approximately 1 ℃. ④When substandard products are returned to the distillation system due to changes in distillation conditions, their quantity should not exceed 20% of the total feed material, and an appropriate amount of water must be added to maintain stability in the composition of the material entering the tower. In short, during operation it is necessary to rely on the vapor-liquid equilibrium relationship in the methanol-2-water system in order to maintain a balance of materials and heat required by the pressurized tower and the atmospheric pressure tower at all times. During adjustment, it is necessary to comprehensively analyze the interrelationships among various parameters, making fine and gradual adjustments as well as advance adjustments to ensure the normal operation of the distillation process. Op in ion s aboutM ethanol Rect if ica t ion *E Shu2sheng (Yankuang L unan Chem ical Fert ilizer P lanт, 277527) Abstract　The paper describes the principle of the three-methanol rectifying tower process, analyzes the material and heat balance of pressurized rectifying towers and atmospheric rectifying towers. Some suggestions regarding operation are also presented. Key words　methanol rectification, pressurized tower, atmospheric tower, material and heat balance ·Brief News· The polyoxymethylene project has recently been officially established in Nantong Economic and Technological Development Zone. The 60,000 tons per year polyoxymethylene production project of PTM Engineering Plastics (Nantong) Co., Ltd. has been approved and is now operational in Nantong Economic and Technological Development Zone. It is reported that this project is the largest foreign-invested project brought to Nantong since China’s accession to the WTO. It was jointly funded and constructed by the Japanese company Polyplastics, the Japanese company Mitsubishi Gas Chemical (MGC), and the American company Ticona. The total land area covered by all the construction projects is 240,000 m2, of which the first phase covers 60,000 m2. The investment amount for this phase is 140 million US dollars, and the project is expected to be completed and put into operation in 2005. 　April 2002　　　　　　　　　　Xie Shusheng et al.: Insights into Several Issues in Methanol Distillation　　　　　　　　　· 45 · © 1994-2007 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net